Why is breathing for singing sometimes so difficult?
We all breathe anyway, all the time, without even thinking. However, a singer can sometimes find him/herself in a tricky situation concerning these natural body functions.
Unfortunately, breathing for singing easily becomes too neurotic. Knowing its importance, you might concentrate too much on your breathing! Suddenly the natural automatic thing can become strangely difficult, too conscious. Abdominal muscles can begin to tense, there isn´t enough air to finish the phrase, the sound gets stuck in the throat – and all this, in spite of trying to support like crazy! Aah, there you are: in the middle of a typical singer´s crisis. Too much of a good thing can give the opposite effect.
I´m not going to describe the whole breathing and support system now. Let me just bring up a few points that might help if this issue troubles you.
First, remember that breathing really is natural and you  only need air for your next musical phrase, just as when you speak a sentence. Think what you are going to sing. Then relax your abdominal muscles and LET the air come in without sucking, just silently opening your vocal folds. No noise while inhaling! Feel the expansion of your body but  don´t overdo it. Begin the phrase with your personal musical intent, making sure you are not pushing air but keeping the nice expanded feeling in your ribs and your back. No need to tense your belly  because that only creates more pressure. Instead, try to feel the expansion of your waist inside, thus engaging your transversus abdominis, the inner core muscle connected to your diaphragm. These muscles form a firm basis for your support, situated on both sides of your waist as the innermost layer of your abdominal muscles. You can feel them also in your back.
Sometimes it almost helps if you “let go” of your old support because you might be tensing your abdominal muscles too much, especially those of your upper abdomen. In my opinion, that it a big problem especially for women who have been trying to keep their belly in afraid of a bulging waist. Or if you are a ballet dancer, do check this. With this kind of mistaken “support”, tension can build up, inhalation becomes a struggle, no amount of air seems to be enough, the breath really never goes deep down  because the diaphragm suffers from restricted movement.
Practice tip: pretend to “sing” your phrases hissing gently through the music with “sss”. No pitch or text is needed for this experiment.  What did you experience in your body? You will probably notice that there was enough air, the air flow was even and you hopefully didn´t get too tense.  Your vocal folds are not going to need any more air flow or pressure for singing.  Thus you should be able to  sing the same phrase similarly, given that the folds close properly.
The singer´s body should be active but at the same time extremely sensitive, mirroring the feelings and emotions of the musical meaning and the sung text.